1. Field of the Invention
The present invention relates to integrated electronic circuit technology. More particularly, the present invention relates to electrically programmable antifuse elements and to methods for measuring the resistance of programmed antifuse elements.
2. The Prior Art
Antifuses typically consist of a metal/dielectric/metal sandwich where the dielectric can be any material that is not a conductor, for example amorphous silicon. Once an antifuse has been programmed into a conducting state, it may later become open or non-conductive. It has been found that this is more likely to occur when the resistance of a programmed antifuse is higher than desired or expected as measured shortly after programming. As such, the reliability of a programmed antifuse is more readily guaranteed when the resistance of the programmed antifuse is known.
In the prior art, however, determining the resistance of a programmed antifuse has been a difficult task. One reason this difficulty arises is due to the fact that access to the programmed antifuse is typically had via the same circuit path which was used to program the antifuse. Those of ordinary skill in the art will recognize that the programming path is in series with the programmed antifuse and is a relatively high impedance path. The total resistance of this path may in fact normally be on the order of several hundred ohms, while the programmed antifuse generally has an impedance on the order of one hundred ohms.
Because of the location of the antifuse in this resistive network including the programming path, it has been extremely difficult to determine the actual resistance of an individual programmed antifuse in a normal integrated circuit. At best, a method based upon data taken from mechanical circuit probes at discrete programmed antifuses and a knowledge of the current which has passed through the devices have been used to provide a reasonable guess. Taking this data is possible only prior to the packaging process for the integrated circuit since it is not practical to measure the resistance of individual antifuses once the circuit has been encapsulated in a package.
It is therefore an object of the present invention to provide apparatus and a method for accurately determining the resistance of a programmed antifuse in an integrated circuit.
It is a further object of the present invention to provide apparatus and a method for accurately determining the resistance of a programmed antifuse in an integrated circuit after the integrated circuit has been packaged.